Soybeans: black root rot increases

For some east Arkansas growers, a switch from cotton[4] to soybeans[5] has had serious consequences. The culprit is black root rot, a disease that has caused serious yield losses in some Mid-South fields.

“There are several things to talk about with regard to black root rot,” says Scott Monfort, Arkansas Extension plant pathologist, who first discovered black root rot in soybeans in 2008. “First, it’s driven by weather and environmental conditions.”

Prior to its discovery in Arkansas by Monfort and colleagues, black root rot — caused by the Thielaviopsis basicola fungus attacking plant roots — had only been confirmed on soybeans in regions north of the Mid-South.

“That’s because of the weather. Environmental conditions here at planting, typically, are a lot warmer at planting and black root rot doesn’t tend to cause problems.”

However, as Mid-South farmers well know, the last couple of planting seasons were extremely cool and wet.

“Those conditions have been common up through May,” says Monfort. “That’s one of the reasons why this disease has begun to show up on soybeans.”

Does that mean we’ll see black root rot only once in a while, when weather conditions are right? “That could be the case. But where it has shown up, it’s proven to do quite a bit of damage.”

Black root rot is a common pathogen on cotton in Arkansas. It typically hits the fields planted first, in April. In Arkansas, “we don’t plant too many beans in April. That’s why once cool, wet weather stretched into May, this disease was able to cause problems.”

In the Mid-South, the disease has yet to be found outside east Arkansas. In 2008, Monfort found it in only one field in one county. Since then, black root rot has been found it in multiple soybean fields in over 10 counties.

In seedlings, black root rot shows up within the first month of growing. One of the problems is symptoms of the fungal disease often mimic those of other maladies. Monfort says those can include: soil compaction, nutrient deficiencies, and plant stress issues (like drought).

“Three typical, above-ground symptoms often observed with black root rot are severe stunting, stacking of the nodes, and chlorotic spots that form along the veins of the leaves.”

To best distinguish the disease, examine the root system of the plant. “The primary diagnostic characteristic of black root rot is blackened, deformed roots. The blackened roots are the result of fungus infection and colonization of cortical tissue which eventually leads to root necrosis.

“Confirmation of the fungus can be achieved from plating out infected roots on specialized media specific to Thielaviopsis basicola. A positive result will show growth of the fungus out of the infected root tissue as dark-colored round colonies on the media.”

What about managing the disease? There are some seed treatments that can help with this disease in cotton, says Monfort. “Most of the time, we have such a treatment in packages for control of seedling diseases. But even there, while it does well, it doesn’t control the disease 100 percent. This disease is pretty resilient and can cause a lot of damage.

“In cotton, we don’t usually see a severe problem with black root rot unless rootknot nematode is in the mix. Rootknot nematode kind of enhances the problem more than when the pathogen is alone. The nematode allows it an extra avenue to enter the roots.”

However, in soybeans, black root rot has been “rather devastating” even in the absence of nematodes. “We’re trying to do research work on that. We want to know how bad this disease can be by itself and also in the presence of a nematode.”

Research trials in 2009 (which are continuing in 2010) — were held in both field and greenhouse settings “to understand the potential of this disease on soybeans and to generate possible control recommendations. The research trials evaluated varieties for potential resistance as well as seed treatments for potential control. So far, greenhouse trials have shown black root rot to have a significant impact on soybean seedling survival — 38 percent of plants survived in infested soil compared to 71 percent of plants in non-infested soil. All surviving plants grown in infested soil were found positive with black root rot.”

Growers who suspect they might have a problem with black root rot in early-planted soybeans should contact their local Extension agent.